The information in this document is based on these software and hardware versions:

Cisco 2503 routers

Cisco IOS Software Release 12.2(24a) running on all the routers

The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.

Before Cisco IOS Software Release 12.1.3, when redistributing connected routes into OSPF, connected networks included in the network statements under router OSPF advertised in Type-1, Type-2, or Type-3 link-state advertisements (LSAs) were also announced in Type-5 LSAs. Memory is required to store those Type-5 LSAs. If the router originates a type-5 LSA for every connected network, even one over which OSPF runs natively, then a large number of redundant Type-5 LSAs is created. The storage also requires a CPU to process the LSAs during full or partial Shortest Path First (SPF) runs and to flood them when some instability occurs.

The advertisement of the connected networks through Type-5 LSAs may also cause problems in certain situations, when routes learned through a different protocol are redistributed into OSPF. Refer to Common Routing Problem with OSPF Forwarding Address for more information.

The following example shows the creation of the Type-1, Type-2, Type-3, and Type-5 LSAs. Use the show ip interface brief command to see all the directly connected networks.

You can see in the above output that there are Type-5 LSAs generated for each of the networks connected to the router. In addition to the Type-5 LSAs, the three networks are also advertised using the router LSA (Type-1) created, as shown below. You can use the show ip ospf database router command to display information only about the router LSAs.

In Cisco IOS Software Release 12.1(3) and later, the Type-5 LSAs are no longer created for connected networks included in the network statements under router OSPF. Using the same router as above, now running Cisco IOS version 12.2(2), you can see that only router LSAs are created:

This document demonstrates the different behavior of redistributing connected routes into OSPF. The change in behavior began in Cisco IOS version 12.1(3). Refer to Bug ID CSCdp72526 (registered customers only) in the Bug Toolkit for more information.